Polymers such as polyesters, liquid crystalline polyesters, and polyamides are commonly compounded with fibers, such as glass or carbon, to produce reinforced composites. The mechanical properties of the molded composite determines the applications in which these materials are used.
The mechanical properties of a polymer composite are determined by volume averaging the individual mechanical properties of the fiber and polymer. The contribution of the fiber to the composite properties is dependent on the material that comprises the fiber as well as the final aspect ratio (length/diameter).
It is known to combine reinforcing agents, such as glass or carbon, into polymer matrices. The dependence of the fiber length on the molecular weight is also known. Processes are known to produce long fiber reinforced polymer composites; however, these processes do not utilize standard extrusion compounding technology.
PCT application WO 94/26814 discloses glass fiber reinforced polyester resin. A polyfunctional compound is added during melt-shaping. The shaped article can then be subjected to solid state polymerization.
U.S. Pat. No. 4,163,002 (Pohl et al.) discloses the solid state polymerization of a glass reinforced poly(1,4-butyleneterephthalate) or poly (1,3-propyleneterephthalate). The additives, such as stabilizers and flame retardants, are compounded into the polymer mixture prior to solid stating.
PCT application WO 92/17552 discloses increasing the solid state rate of a polyester through the addition of an upgrading additive into a polyester melt blend, granulating the blend, and then subjecting the granulate to crystallization and an upgrading reaction in the solid state.
The properties of the polymer depend on the molecular weight of the polymer as well as the polymer type. High molecular weight polyesters may be prepared by solid state polymerization, as for instance in (polyethylene terephthalate) (PET) packaging applications. Solid state polymerization may also be used to prepare liquid crystalline polyesters and nylon. Subjecting a polymer to solid state polymerization prior to compounding with a reinforcing fiber is ineffective in increasing the performance of the composite material. The higher starting molecular weight of the solid state polymerized polymer causes high shear forces during compounding of the polymer which results in a faster rate of degradation and thus greater loss in molecular weight. The higher molecular weight polymer results in a higher melt viscosity which leads to more fiber breakage, leading to inferior properties. The loss in molecular weight and the increased fiber breakage both lead to inferior properties.
Solid state polymerization is a well-known technique for building up the molecular weight of a polymer in the solid state. One conventional solid stating operation takes place in a plug flow reactor in which polymer pellets are introduced into the top of a tall cylindrical vessel and removed from the bottom at the same rate. During the residence time in the reactor, which is commonly on the order of 4-18 hours, the pellets reach high temperatures, generally in the range of 200-250 C. The particular temperature is dependent on the polymer being used. Solid state polymerization is disclosed in U.S. Pat. No. 4,064,112, which is incorporated in its entirety herein by this reference.
Many additives that are often useful in fiber reinforced composites are not compatible with the high temperatures over a period of time that are required for solid stating. There exists a need for compositions and methods that provide excellent properties obtained during solid state polymerization, while avoiding the degradation of additives during the solid state processing of the polymers.